General:
* कौन सा? (kaun sa?) - This is the most common and versatile way to say "which one" in Hindi. It can be used for both masculine and feminine objects.
* कौन सी? (kaun si?) - This is used for feminine objects.
* कौन से? (kaun se?) - This is used for plural objects.
Examples:
* कौन सा किताब? (kaun sa kitaab?) - Which book? (masculine)
* कौन सी कलम? (kaun si kalam?) - Which pen? (feminine)
* कौन से फल? (kaun se phal?) - Which fruits? (plural)
Other options:
* कौन सा वाला? (kaun sa wala?) - This literally translates to "which one वाला (wala)?" and is a more informal way to say "which one."
* कौन से वाला? (kaun se wala?) - This is the plural form of "kaun sa wala?"
Remember that the choice of word depends on the gender of the noun you're referring to.